HKSI Paper 1 Participating in the Hong Kong exchanges question: CAS reference price

HKSI Paper 1Participating in the Hong Kong exchanges

Shares of Sai Kung Holdings, a CAS security, show nominal prices of HK$50.20 at 3:59:00 p.m., HK$50.10 at 3:59:15, HK$50.40 at 3:59:30, HK$50.30 at 3:59:45 and HK$50.10 at 4:00:00. What is its CAS reference price?

A HK$50.20, the median of the five snapshots
B HK$50.25, the midpoint of the highest and lowest snapshots
C HK$50.22, the average of the five snapshots
D HK$50.10, the last nominal price at 4:00 p.m.

Show the answer and explanation

Answer: A. HK$50.20, the median of the five snapshots

In the Reference Price Fixing Period (4:00 to 4:01 p.m.), the system takes five snapshots of the nominal price at 15-second intervals from 3:59:00 p.m. and uses their median as the CAS reference price. Ranked, the snapshots are HK$50.10, 50.10, 50.20, 50.30 and 50.40, so the median is HK$50.20. The reference price then sets the ±5% Stage 1 price limit for the auction. HK$50.22 is the mean, not the median; HK$50.10 is only the last snapshot; and HK$50.25, the midpoint of the range, is not a method HKEX uses. Source: HKEX 'Trading Mechanism of Closing Auction Session (CAS) in the Securities Market' (version 10 April 2026) sections 3 and 4 (as at October 2026). Source: HKEX 'Trading Mechanism of Closing Auction Session (CAS) in the Securities Market' (version 10 April 2026) sections 3 and 4 (as at October 2026).

Common mistake

Candidates take the last price before 4:00 p.m. or the average. The reference price is the median of five snapshots.

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